1. Field of the Invention
This invention relates to a hologram for security markings, to a method of making the same and to an apparatus and method for reading light reflected from the hologram.
2. Description of the Related Art
The invention generally relates to holograms for security markings for documents, commercial goods and packaging. Holograms have a high degree of security because they are difficult to reproduce and hard to simulate by conventional printing methods. Numerous prior art patents are known which disclose the use of visually discernible holograms for authenticating commercial goods, packages and documents of value such as bank notes, bonds and vouchers. For example, GB-A-2092952 discloses an arrangement for identifying an article which comprises producing a holographic image on a sheet or film and embodying the sheet or film with the image into the article or its packaging. The holograms most suited for use in product authentication are those produced by embossing into thermo-plastic and various known methods are described in U.S. Pat. No. 4,773,718. A particular method of making an embossable hologram is disclosed in U.S. Pat. No. 3,565,978 having the steps of forming a surface deformation pattern in an insulating thermo-plastic layer, treating the surface to harden or toughen it, forming a first generation replica by applying a curable casting material, curing the casting material to form a negative replica to form a first generation negative replica, forming a second generation replica that is cast from the first generation replica, transferring the second generation replica to a support device to form a final positive replica of the image, silvering the second generation replica, electroforming a metal surface on the silvered second generation replica, separating the metal layer from the second generation replica to form a third generation metal negative replica and pressing a thermo-plastic preform against the metal replica to form a fourth generation final positive replica of the original image.
A further example of the use of embossed holograms as security devices is described in GB-A-2129739 which discloses incorporating a hologram into hot stamping foil so that the hologram may be intimately bonded to a substrate in such a way that the hologram cannot be removed without destruction of the hologram.
All the above disclosed prior art patents relate to holograms which are embossed and which may be used as a security means, but which diffract incident white light into a plurality of spectral colours. Such a hologram is sometimes known as a "Benton" hologram or a "rainbow" hologram because of the manner in which the holograms diffract incident white light into its spectral components and it is believed that the origination of such a hologram was first disclosed by Benton in U.S. Pat. No. 3,633,989.
The bright diffractive colours associated with a rainbow hologram are usually deemed an advantage because the colours are easily discernible in ordinary white light, such as the light from the sun or a spotlight or domestic lamp. However, there are instances where the discernible, visible hologram image is not an advantage and a more discrete, covert, form of authentication is preferred.
It is a first object of this invention to provide a hologram for security markings and, in another aspect, to provide a method of making a hologram for security markings. Features of this invention include apparatus and a method of reading said hologram.